1. Field of the Invention
The present invention relates to an organic electroluminescent device having a good luminescent state and excellent luminous efficiency.
2. Description of the Related Art
The organic electroluminescent device (hereinafter, organic EL device) is an emissive device using a principle in which a fluorescent material emits light using recombination energy of holes injected from an anode and electrons injected from a cathode by electric field application. A representative example of a study on this organic EL device includes a low voltage driving organic EL device using a stacked device, which has been reported by C. W. Tang et al (e.g., see C. W. Tang, S. A. VanSlyke, Applied Physics Letters), Vol. 51, p913, 1987, etc.). With the development of this stacked device, the luminescence property of the organic EL device was significantly enhanced. And, as this high performance organic EL device has been developed, in recent, study and development on the organic EL device have been actively made to practically use it.
A two-layer structure by Tang et al. uses tris(8-quinolinol)aluminum (AlQ) for an emitting layer, and a triphenyldiamine derivative (TDP) for a hole transporting layer. This two-layer structure exhibits excellent characteristics because it has high efficiency in hole injection into the emitting layer, it further has high production efficiency of excitons formed through recombination by blocking the electrons injected from the cathode, and the formed excitons can be sealed in the emitting layer. Further, as an example of enhancing this two-layer structure, a three-layer structure of a hole transporting (injecting) layer, an emitting layer, and an electron transporting (injecting) layer has been reported. This three-layer structure has been known as a representative structure of an organic EL device along with the above-stated two-layer structure of the hole transporting (injecting) layer and the electron transportable emitting layer. Furthermore, what is needed in such a stacked device is to enhance the recombination efficiency of holes and electrons. To meet this need, numerous studies have been made.
By the way, because the organic EL device has a high response time and is an self-emissive device, the organic EL device is expected to be practically used as a high definition display for a portable terminal or a television set, but it is considered that it is necessarily required to enhance the light-emitting efficiency of an organic EL luminous body to realize a high definition organic EL display into products. Accordingly, a need to enhance the light-emitting efficiency of the organic EL device will be hereinafter described in detail.
First, considering a carrier recombination principle in the organic EL device, electrons and holes injected from an electrode to an emitting layer become electron-hole pairs by Coulomb's interaction in which some of the pairs become singlet excitons and the others form triplet excitons. The production ratio becomes 1:3 depending on quantum mechanical density. That is, if there is no phosphorescence observed from the triplet state, the quantum yield in luminescence becomes a maximum of 25%, which indicates that only the maximum efficiency of 25% is obtained from the organic EL device. Further, there is a problem with the organic EL device in that, because the refractive index of the luminous body affects the device, light at an exit angle larger than a critical angle causes total reflection and accordingly cannot be emitted to the exterior. That is, if the refractive index of the fluorescent substance is 1.6, only 20% of the total amount of emitted light is effective. Further, a summed production ratio of the above-stated singlet (production efficiency: 25%) becomes about 5% of the total, and the light-extraction efficiency of the organic EL device is significantly degraded (for example, see ┌The phenomenon and trend of an organic electroluminescence┘) by Tetsuo Tsutsui: Monthly Display, Vol. 1, No. 3, p 11, September 1995). For this reason, it is necessarily required to enhance the light-extraction efficiency. Otherwise, such fatal degradation may be caused in the organic EL device.
Therefore, several methods have been studied aiming at developing technologies on an inorganic EL device as solutions for enhancing the light-emitting efficiency. Examples of such studies include a method for allowing a substrate to hold light focusing capability (for example, see Japanese Patent Laid-open No. Sho 63-314795) and a method for forming a reflective surface at a side of a device (for example, see Japanese Patent Laid-open No. Hei 1-220394).
Furthermore, as a conventional configuration, there is disclosed, in Japanese Patent Laid-open No. 2001-60495, an organic EL device sequentially comprising a substrate, a color filter layer, a barrier layer, a hole injecting electrode, an organic emitting layer, and an electron injecting electrode, wherein silicon oxide is normally used for the barrier layer. There is also disclosed, in Japanese Patent Laid-open No. 2002-260845, an organic EL device comprising an underlying layer of SiO2 and the like formed over an entire surface of miniature lens, and an ITO film formed on the underlying layer.
However, the methods listed herein is usable for a large sized substrate, while it has a problem that, in a high definition display configured to have a very small pixel area, it is difficult to manufacture lens having a light focusing capability or to form a reflecting surface that is a side surface. And, in an organic EL device comprising an emitting layer with a thickness of several microns or less, it is very difficult to form a reflecting mirror on a side surface of the device even with a super fine processing technique, and the reflecting mirror may be formed but manufacturing cost significantly increases, which greatly obstructs a practical use.
Meanwhile, an example of introducing a flat layer, which is used for an anti-reflective film, having an intermediate value of respective refractive indexes of a substrate glass and a luminous body between the substrate glass and the luminous body has been reported, as a study example on a method different from the method having the light focusing capability and the method forming the reflecting surface at a side surface of the device (e.g., see Japanese Patent Laid-open No. Sho 62-172691). However, it is considered that forward light-emitting efficiency in this method may be enhanced but cannot prevent total reflection. That is, there is a problem with the principle of the anti-reflective film that even though the principle is effective in a luminous body with a large refractive index such as an inorganic EL, it cannot significantly enhance the light-emitting efficiency in an organic EL device that is a luminous body with a lower refractive index as compared to the inorganic EL device.
Although numerous studies on the light-emitting efficiency of the organic EL device have been reported as stated above, they do not yet meet desired performance, thus a countermeasure of a new concept is required.
Accordingly, as a new method that enhances the light-emitting efficiency, a study example in which an optical element such as a diffraction grating or the like is formed on a substrate (for example, see Japanese Patent Laid-open No. Sho 62-172691) has been reported. It is considered that this method is effective to enhance the light-emitting efficiency of the organic EL device. On the other hand, in case of the diffraction grating type organic EL device presented herein, high light emitting efficiency is possible but it is difficult to manufacture the device. In particular, even with up-to-date manufacturing techniques, it is difficult to accomplish a groove processing process in which a diffraction grating is buried, and a planarization technique in which a substrate is subject to the planarization prior to formation of a device, and also it is difficult to form electrodes on a substrate, suppress the deviation of luminescence property in a surface of a substrate.